Int 21 Fn 4403  U - Ms-dos 7.0 - Config$ Device - Set??? Configuration In  [D]

   AX = 4403h
   BX = file handle for character device "CONFIG$"
   CX = number of bytes to write
   DS:DX -> buffer containing ???

Return: CF clear if successful
       AX = number of bytes actually written
   CF set on error
       AX = error code (01h,05h,06h,0Dh) (see #1366 at AH=59h/BX=0000h)

Note:  IOCTL Output to CONFIG$ is only available while IO.SYS is booting
     the system; once CONFIG.SYS has been processed, this call always
     succeeds, ignoring any input

See Also: AX=4403h"CONFIG$"